This invention relates to an ink filter for use in an inkjet printer and, in particular, to a filter which removes trapped air from inside the inkjet printhead.
It is desirable for some printers to be able to print graphics or text on vertical surfaces, such as sides of transit cases and the like, and in order to achieve this, a multi-nozzle inkjet printhead must be orientated such that each of the nozzles or groups of nozzles within the head are vertically aligned above the other. Each nozzle or group within the printhead may have its own ink inlet and associated filter element, particularly when the printer uses hot-melt ink.
Such a filter element is shown in FIG. 1 in which a filter housing 1 is provided with an inlet two and an outlet 3. A filter element 4, such as a filter mesh, is provided within the filter housing 1 and between the inlet 2 and the outlet 3.
Whilst such an arrangement is beneficial in allowing a comparatively high surface area of filter material in a relatively small housing, an unwanted side effect is that, as the filter mesh is formed in an approximately conical shape, air bubbles 5 may be created as described below. The air bubbles become trapped in the filter, on one side by the filter mesh 4 and on the other side by the flow of ink 16.
The capillary action of the ink in the pores of the filter mesh results in the filter mesh having a xe2x80x9cburst pressurexe2x80x9d, i.e. a pressure below which air cannot be forced through the mesh. With fine filters as commonly used in inkjet printheads, this burst pressure is significant. Accordingly, as ink, but not air, can flow across the mesh, small bubbles of air coalesce to produce an air bubble trapped as shown in FIG. 1. This is a particular problem for drop-on-demand printheads as it prevents the ink feeds to the head from being properly primed. The present invention is aimed at overcoming this problem.
According to the present invention, there is provided a filter for use in an inkjet printer comprising:
a housing having an inlet and an outlet;
a filter element tapered towards the outlet for, in use, removing unwanted substances from ink flowing through the housing; and
a bleed tube, having an inlet disposed on the inlet side at the tapered end of the filter element, for removing trapped air from the inlet side of the filter element.
Thus, the present invention provides a filter in which any trapped air can be quickly and simply removed such that the ink feeds to the inkjet printhead is not interrupted.
The filter element may be conical, hemispherical, frusto-conical, pyramidal or wedge-shaped.
Preferably the bleed tube enters the filter housing upstream of the filter through the fluid inlet.
Alternatively, the filter housing may be provided with an additional opening through which the bleed tube extends to remove unwanted air.
The bleed tube may be vented to the atmosphere or the system may be purged by applying a positive pressure to the bulk ink inlet or a negative pressure to the bleed tube. The use of the bleed tube provides an escape route for the trapped gas bubbles and this greatly improves the priming effectiveness of the printhead.